...getting Snape to admit Harry is not there for detention and that he cannot give him such during the holidays is a small, but significant victory, don't you think?

Hmmm... I hadn't thought about it that way. I mean, there's not much practical difference between detention and punishment. Harry still has to do what Snape says. And any power he does have over Snape (which you noted in your comment to Switch), he's not aware of. Which is probably the only reason he has it. But anyway.